In electronics, a signal may have multiple states that need to be detected in order to understand the signal. In the most common circuits a signal is binary, having only two possible states. The detection of two states, which are generally high (which may be represented as “1”) or low (which may be represented as “0”), is relatively easy. In a simple example, a signal that is above a certain threshold voltage is high, while a signal that is below a certain threshold voltage is low.
However, some operations use a signal having a greater number of states, such as a three-state, or ternary, signal. In one example, the states for such a signal may be high, low, and high impedance, although three voltage levels may be used in some cases. The introduction of an additional state, while potentially allowing transfer of a great deal more information, creates complications in detection. A simple binary detection circuit will not be able to sense a third state, such as a high impedance state.
In a conventional detection environment, the detection of three states requires either greater complications in detection techniques or additional signal lines to transfer the information for the third state. In a first conventional example, ternary sensing is used, in which three discrete voltage levels are directly sensed. However, such a circuit is not compatible with common binary digital signaling standards, and thus is contrary to the preference for retaining compatibility with common signaling standards.
In second convention example, a pair of binary digital inputs may be utilized. However, while this method actually allows the sense of four states total, it requires that two binary lines be utilized for three states. This method increases the pin count and number of signal lines needed, when the preference is to convey as much information as possible using as few pins as possible.
In a third conventional example, a line may be precharged and then tested. Such a process assumes that the device under test is stimulated, and then the input is sensed. This assumption greatly limits the applicability of the technique to situations in which the user has control of the device driving the input since it does not allow for the continuous sense of the input to determine the high-impedance state.